Originally Posted by MrCooperS

The general thumb of rule for pulleys is:

-The pulley is the BEST bang for buck mod and you can totally feel the difference right away.
-The smaller the pulley the more power/torque on the lower end of RPM
-The smaller the pulley the less torque you will generate at higher RPM
-The smaller the pulley the more heat will be generated at higher RPM
-15% is the most popular pulley choice
-There is no proven facts that a 16% 17% or 19% does any harm to your SuperCharger
-With any pulley upgrade, you should always get colder spark plugs
-It is recommended that a 15% be chosen for heavy track use
-It is recommended that a 17% be chosen for daily driving and light track/auto-x use
-It is recommended that a 18% be chose for only daily driving
-Most pulleys are the same so it doesn't matter what brand the pulley is.

Of course there are exceptions to every case. If I'm wrong, others please chime in.